The present invention relates to a control system for controlling the operation of a vehicle parking brake, and more specifically, an electrohydraulic control system for controlling the operation of a spring-applied, pressure-released parking brake.
It is known to provide a vehicle with a spring-applied, pressure-released parking brake that is separate from the service brakes. Such a parking brake can be engaged with a drive train component to prevent movement of the vehicle without having to shut down the engine, and such a parking brake will automatically disengage upon engine shutdown. However, in the event of an engine failure or other mechanical failure, there must be some provision for releasing the parking brakes so that the vehicle may be pushed or towed to a place where repairs may be made.
For example, U.S. Pat. No. 3,999,075 issued to P. R. Johnson et al. on Dec. 21, 1976, discloses a brake control system that includes combined air and hydraulic circuitry. The parking brakes are normally released by pressurized fluid from an engine driven pump. When the hydraulic pressure drops below a predetermined level, pneumatic pressure from an accumulator allows normal brake operation. In the event of a mechanical failure, a parking brake valve communicates air pressure from the accumulator to a master cylinder, which, in turn, supplies pressurized hydraulic fluid to release the brakes. However, this system requires extensive hydraulic and pneumatic circuitry.
An entirely mechanical parking brake spring override mechanism is disclosed in U.S. Pat. No. 4,245,724, issued to H. E. Beck on Jan. 20, 1981. This mechanism includes a bolt or a plurality of bolts which may be inserted through an opening in the outer housing of the brake to engage a threaded aperture in a member that is attached to the brake piston. Rotation of the bolt draws the piston away from its engaged position, against the bias of the spring, and releases the braked members. However, this system requires access to the parking brake in order to release it. This is inconvenient because the parking brake is often located in an area that is not easily accessible. Also, special tools are required to install and remove the bolts, and such tools may not be available at the location of a failure.
Some production row crop tractors have a parking brake which includes a park pawl which is coupled to a shift lever on the tractor control console. When the shift lever is in the park position, teeth on the park pawl engage a reduction gear in the transmission in the park position, thereby locking the drive train and preventing vehicle movement. However, such a parking brake system requires a mechanical linkage between the shift lever and the park pawl. With the advent of electronically controlled transmissions, it would be desirable to provide a parking brake function which does not require mechanical linkages. For example, if electronic shift controls are mounted on an armrest of a pivoting vehicle seat, then it would be impractical to connect mechanical linkages to such a pivoting platform.